Plants for the manufacture of sulphuric acid involving either the burning of elemental sulphur or oxidation of metal sulphides to produce sulphur dioxide for subsequent oxidation to sulphur trioxide followed by absorption into sulphuric acid are very large generators of process heat. This process heat comes from the exothermic burning or absorption processes and is generally used for many purposes, such as the heating of gases or raising steam.
The SO.sub.2 oxidation is carried out in a series of uncooled catalyst beds of a catalytic converter with heat being removed between beds and before the SO.sub.3 -containing gases are passed to absorber(s) for SO.sub.3 removal. In sulphur burning sulphuric acid plants the bulk of the heat removed from the SO.sub.3 -containing gas is transferred into steam systems with only limited pre-heating of process or other gases. In plants using an SO.sub.2 gas source, the heat is almost completely used to pre-heat incoming cold, dry SO.sub.2 feed gas, or, in addition, in the case of the so-called "double absorption process", the cold SO.sub.2 -containing gas returning from the gases in the first or intermediate absorber. Where surplus heat must be rejected from such plants, often the heat is rejected to either atmospheric air or to plant tail gas in special exchangers designed for this purpose.
Inter-bed exchangers used in such processes are, typically, known as Hot, Intermediate, or Hot IP Exchangers. The Hot Exchanger is normally associated with cooling of the hot gas leaving the first catalyst bed and the other exchangers with cooling of the gases between beds 2 and 3. In all of these exchangers the heat is transferred to colder SO.sub.2 -containing gases which then pass either directly or through other exchangers to catalyst beds. The gases leaving catalyst beds en route to absorption steps are normally cooled by heat transfer with cold SO.sub.2 -containing gases from either an acid plant main blower or an Intermediate absorber. It may also be cooled by heat transfer with air in what is known as "SO.sub.3 Coolers" or "Air Heaters" or by plant tail gas, in which case the apparatus becomes known as a Tail Gas Heater.
Classic heat transfer between SO.sub.2 - and SO.sub.3 -containing gases uses counter-current shell-and-tube heat exchangers in which one gas flows through the tubes of the exchanger and the other gas flows through the shell space as directed by baffles within the shell space. These exchangers are, typically, quite large and for colder duties are made of carbon steel. More recent plants use stainless steel as a construction material for hotter duties such as involving the cooling of the hot SO.sub.3 gas leaving beds 1 or 2 where the SO.sub.3 gas is hottest. In most large plants, the exchangers are fabricated on site as they are too large and heavy to allow of reasonable transportation. Tubes of exchangers are arranged in a number of different ways with baffles to match the tubing layout. In some cases the tubes are distributed throughout the shell space and either single or double-segmental baffles have been used. In other designs, the tubes are arranged in the form of annular bundles wherein gas flows radially through the bundle from an open core to a tube-free outer annulus and returns as required. The number of passes across the bundle and the tubing layout will depend on the size of gas flows, thermal efficiency needed and the pressure differences available to cause flow through the shell space.
In plants where heat is rejected from the process to atmosphere, early plants used simple bare gas ducting to cool gases between beds, such as, for example, between a third and fourth catalyst bed in a small single absorption plant. Such apparatus was simple but not effective in rejecting large quantities of process heat. Induced draft heat exchangers were subsequently used to reject significantly larger quantities of heat. The pressure difference available using stack draft was, however, small and, accordingly, fans or blowers were introduced to provide adequate pressure differences and allow the size of such equipment to have reasonable physical dimensions. Where an exchanger handled air that was used elsewhere in process, SO.sub.2 -containing gas, or plant tail gas, the main acid plant blower provided the driving force for gas flow and separate blowers were unnecessary. Where air was heated and rejected directly to atmosphere a separate air blower was used.
Each of the exchangers described hereinbefore was based on counter-current heat transfer with the two gases entering at opposite ends of the exchanger. Problems exist if the metal of the exchanger becomes too cold or too hot. Gas streams found in sulphuric acid plants normally contain condensible compounds, such as small quantities of sulphuric acid vapour, either from entrained acid from drying operations, from reaction of SO.sub.3 formed in the reaction with moisture from inadequate drying, or from hydrocarbons present in the elemental sulphur if sulphur is used. As a result, there is the possibility of sulphuric acid condensation from such gases when the temperature of the metal exchanger falls below the condensation temperature. This condensation produces significant corrosion. Although the condensation temperature is normally not a factor in the hotter exchangers, it is a problem in colder exchangers such as Cold or Cold IP Exchangers, SO.sub.3 Coolers or Tail Gas Heaters.
Where the condensation risk is serious, special measures are often taken to keep metal temperatures above the minimum at which condensation takes place. One such technique is to recycle hot air from the exit of a SO.sub.3 cooler back to its inlet. This corrective action is widely used, but requires a much larger fan and heat exchanger and, hence, larger capital and operating costs. Where tail gas is being heated, there is little prospect of a recycle stream without the need for a separate fan and the operator is, thus, normally forced to accept any condensation that results. Such equipment is therefore very dependent on the quality of the drying and mist elimination equipment upstream.
Conventional exchanger designs result in large exchangers having high flow resistance due to the large gas flows involved. The large exchangers also often have significantly different thermal expansions between adjacent parts of the exchangers. Cracked tube sheets, broken tube-to-tube sheet joints and leaks can result from excessive differential thermal stresses in such units.
The shell and tube exchanger having a shell full of tubes has fallen into disfavor in the last two decades as the shell and adjacent tubes have significantly different thermal expansions and generate excessive stresses on tube-to-tube sheet joints or on tube sheet-to-shell joints. Heat transfer varied significantly from tube-to-tube in the shell space and the unit used many more tubes than necessary. Baffle arrangements included single and double segmental baffles with the problem being common to both baffle arrangements.
In an alternative design, the tubes of the exchanger are confined between chords with open dome spaces on each side of the tube bundle for gas flow between cross-flow passes. With single segmental baffles, this arrangement provides for gas transfer from one shell pass to the next in the dome space where no tubes are located. Better heat transfer is provided as all of the tubes are located in a zone where good gas flow is assured but pressure drop in the shell space is high. This design has also been used with double segmental baffles. In the double segmental baffle variation, gas flows either around and parallel to tubes in the central portion of the bundle or in the two dome spaces which are free of tubes. The gas flows from the edge of the bundle to the centre of the bundle and then back. While there are variations in tube temperature as not all tubes are subjected to the same shell side gas flow, this exchanger design allows smaller shells to be used which often offers a cost advantage.
A further alternative design uses an annular dome space next to the shell and an axial dome space. Both dome spaces are free of tubes. Gas flows from pass to pass in the dome spaces and radially across the bundle. This design uses several shell passes, offers better temperature distribution across the tube bundle and fewer mechanical problems. It also has significantly less pressure drop and requires less surface than either the single or double segmental baffled units hereinabove described.
In a yet further alternative design, simple crossflow heat transfer has also been used but with mixed success. In this case, the shell side gas enters one side of the shell and flows across the bundle and out of a nozzle on the other side. The tube side gas flows through all the tubes in parallel. This design results in significant differences in tube temperature between the tubes on the inlet shell side gas entry and the tubes on the other side and, accordingly, the exchanger tends to distort towards a "banana" shape. If the temperature difference between the inlet and outlet tube side gas is modest, the differential expansion is modest and the design concept can be quite useful. On the other hand, such an exchanger when located after a first catalyst bed with inlet tube gas temperatures approaching 650.degree. C. can have very strong differential forces and be almost impossible to design mechanically.
Combined combustion furnaces and heat exchangers, commonly known as process preheaters are used in sulphuric acid plants to heat process gases such as air and sulfur dioxide. The preheaters may be used intermittently or continuously. Conventional preheater systems have included horizontally or vertically aligned furnaces which burn fossil fuels such as natural gas or various grades of fuel oils. The heat exchangers have included vertically or horizontally aligned exchangers wherein heat transfer to the process fluid from the furnace gas occurs. Typically, the flow of the furnace gas is countercurrent to the flow of the process gas to enhance transfer of energy and, thus, improve efficiency.
In the manufacture of sulfuric acid, older preheater systems generally comprised a furnace and an associated heat exchanger wherein the furnace was formed of a brick-lined cylindrical shell having an air blower wherein the heated furnace gas exited from the end remote from the air intake and blower. Such fossil fuel combustion furnaces produced a flame extending as much as 3-4 meters in the furnace and only modest efforts were expended to efficiently mix fuel and air. Such furnaces generally required significant periods of time to heat the brick lining to operating temperatures, which brick preheating time affected the operation of the downstream plant.
Such heat exchangers were initially formed of carbon steel, which limited the temperatures that could be generated in the furnace to less than 650.degree. C. Further, these exchangers generally had their heat exchanger tubes vertically aligned and received furnace gas therethrough, while the shell space received the process gas to be heated. These carbon steel exchangers were susceptible to high temperature scaling and, thus, were frequently replaced. In addition, in consequence of the very high temperatures produced in the furnace, it was necessary for large quantities of excess air and/or, larger exchangers to be used. High temperature combustion further increased the risks of formation of unwanted nitrogen oxides and smoke in the preheater exit gas.
Later preheater exchangers were formed of stainless steel and were, thus, able to operate at higher temperatures to provide higher thermal efficiencies. In the sulfuric acid industry, the preheater systems generally had long, horizontal, cylindrical furnaces with either a vertical exchanger or a horizontal exchanger mounted on top of the horizontal furnace. These newer designs also permitted rapid firing in the furnace, incorporated flue gas recycle and air preheating where required to improve thermal efficiency and to minimize formation of nitrogen oxides.
Preheater systems presently in use suffer from a number of disadvantages. It has been found that the shape of the combustion flame of the furnace may be variable in operation and cause inefficient radiative transfer of heat to the heat exchanger. Relatively low intensity combustion results in a longer residence time of the reactants in the furnace which favors the formation of unwanted nitrogen oxides. Further, high temperatures of the metal at the hot end of an exchanger may cause high temperature damage by scale formation and uneven thermal stresses. Yet further, most preheaters of the prior art are not easily adapted to higher energy efficiency by such optional features such as stack gas recycle and air preheating with stack gas.
Thus, heat exchange equipment and processes of the prior art in the sulphuric acid field suffer from one or more of the following problems, viz:
1. The unwanted production of condensed sulphuric acid. PA0 2. The requirement to recycle coolant fluid. PA0 3. The consumption of electric power to move fluids. PA0 4. Relatively large size in capacity in size of equipment is required with associated extra economic cost. PA0 5. Unnecessary thermal stresses due to differential thermal expansion. PA0 6. The need to provide the heat exchange in metal at an operative temperature above the gaseous fluid condensation temperature.
Accordingly, there is a need for improved heat exchanger equipment and associated processes of use in the sulphuric acid plant industry. There is also a specific need for an improved preheater system which does not suffer from the aforesaid disadvantages of prior art preheaters.